1. Field of the Invention
The invention relates to a ceria-zirconia-based composite oxide, a method for producing the composite oxide, and a catalyst for exhaust gas purification including the ceria-zirconia-based composite oxide.
2. Description of Related Art
Composite oxides containing various metal oxides have been utilized as supports, promoters, etc. for catalysts for exhaust gas purification. As a metal oxide contained in such composite oxide, ceria has been used because ceria can absorb and release oxygen in accordance with the partial pressure of oxygen in the atmosphere (has oxygen storage ability). In recent years, various kinds of composite oxides containing ceria are being investigated, and various ceria-zirconia-based composite oxides and methods for producing these have been disclosed.
For example, Japanese Patent Application Publication No. 2011-219329 (JP 2011-219329 A) discloses a ceria-zirconia-based composite oxide including a composite oxide including ceria and zirconia and a method for producing the composite oxide, the composite oxide being one in which the content ratio of cerium and zirconium ([cerium]:[zirconium]) is in the range of 43:57 to 48:52 by mole ratio and which, when heated for 5 hours in the atmosphere under the temperature condition of 1,100° C. and thereafter examined by X-ray diffractometry using CuKα, gives an X-ray diffraction (XRD) pattern wherein the intensity ratio of the diffraction line, at a 2θ of 14.5° to the diffraction line at a 2θ of 29° {I(14/29) value} and the intensity ratio of the diffraction line at a 2θ of 28.5° to the diffraction line at a 2θ of 29° {I(28/29) value} satisfy the requirements (I(14/29) value)≥0.015 and (I(28/29) value)≤0.08. According to this patent document, it is possible to provide a ceria-zirconia-based composite oxide which has, high heat resistance and which even after long-term exposure to high temperatures, can exhibit excellent oxygen storage capacity (OSC). Recently, however, the properties required of catalysts for exhaust gas purification are becoming higher and higher, and a ceria-zirconia-based composite oxide in which the oxygen storage material can be sufficiently inhibited from deteriorating thermally and which can sufficiently exhibit the oxygen absorption/release function from a low temperature has come to be desired.
Meanwhile, International Patent Application No. 2006/030763 (WO 2006/030763) discloses a cerium-zirconium-based composite oxide which is a composite oxide containing cerium and zirconium and which has (1) an oxygen release initiation temperature of 380° C. or lower, (2) an oxygen release amount of 485 μmil/g or larger, and (3) an oxygen release amount at 400° C. of 15 μmol/g or larger, and further discloses a method for producing a cerium-zirconium-based composite oxide which includes mixing a cerium source with a zirconium source in a given proportion, melting the resultant starting-material mixture at a temperature not lower than the melting point, then cooling the melt to form an ingot, subsequently pulverizing the ingot according to need to obtain a powder, successively removing distortions within the powder crystals by heating, and further finely pulverizing the powder. However, the ceria-zirconia-based composite oxide disclosed in WO 2006/030763 is insufficient in the effect of inhibiting the deterioration of the oxygen storage material, does not always sufficiently exhibit the oxygen absorption/release function from a lower temperature, and has insufficient durability.